If PITI + PMI is $4300, you also need to factor in property management, repairs, etc. 

Personally I would call your local investment guru agent or property manager and see what you can do.

My plan is to use FHA, 3.5% dp, seller covers closing costs. 

I’ve heard about ‘gap funding’ for fix & flips to cover down/closing/holding costs in addition to the hard money loan for most of the purchase price & rehab, but how realistic is it to secure that type of funding?

There are even members that will respond here that have ulterior motives, so you shouldn't trust them.Go through the content available here on BP and then start looking for a Property Management Company you can build a relationship with - but, still verify & validate what they do as no one is perfect and even an honest mistake can cost thousands.Some agents may post here to start with them, but 95%+ of agents don't know how to properly identify a rental property to buy.

It's going to cost about $550k to build and it has already appraised well over $800k.

I have an airbnb and the numbers are WAY up this summer, also manage another clients airbnb and both running almost fully booked up this year maybe the registration hurdles have lowered supply. 

Be sure to no only account for the cost of the cash for keys but your additional holding costs while you have to wait to get them out.

Returns in Big Bear never made sense going with traditional management companies. 

Partner Driven added value to the Wildwood Drive deal through strategic marketing and quick transaction management.